Surface mount MELF capacitor

ABSTRACT

The surface mount MELF capacitor of the present invention includes a wire and a conductive powder element electrically connected to the wire. The surface mount MELF capacitor has insulative material surrounding at least a portion of the conductive powder element and the wire extending from the conductive powder element. A first terminal is formed on the surface mount chip capacitor at the first end surface of the wire and a second terminal is formed by being electrically connected to the conductive powder element. The surface mount MELF capacitor of the present invention is created by methods which include the steps of providing a wire and placing conductive powder upon the wire. An embodiment of the present invention feeds the wire in a reel to reel system and electrophoretically deposits the conductive powder element upon the wire.

BACKGROUND OF THE INVENTION

The present invention relates to capacitors. More particularly, thoughnot exclusively, the present invention relates to improved surface mountMELF capacitors and methods for manufacturing the same.

BRIEF SUMMARY OF THE INVENTION

Capacitors exist in the art which are made from a capacitive elementsuch as a tantalum slug or pellet. To create a conventional tantalumslug, tantalum powder is pressed with a binder and then exposed to aprocess for forming a polarized capacitor having a positive end and anegative end. A typical tantalum slug will have an anode comprised of awire extending from the slug and a cathode comprised of a conductivesurface formed at the opposite side of the tantalum slug.

The usual method for making tantalum pellets for use in tantalumcapacitors includes steps wherein tantalum powder is first pressed orcompacted into a pellet. The resulting pressed pellets then undergo asintering process wherein the pellets are heated in a vacuum. Theheating allows the tantalum particles to stick together so they can holda lead wire, which functions as the anode.

Following the sintering process, the tantalum pellet is dipped in anacid solution to form a dielectric film on the outer surface of thepellet and the particles within the pellet which is typically tantalumpentoxide. The pellet and the particles within the pellet are thensubsequently coated with various other metal-containing materials whichform the cathode.

These capacitors have the anode and the cathode attached to a circuitboard by connection wires.

Modern methods of mounting components use the possibility of solderingthe components directly to conductor tracks of printed circuit boardswithout the use of connection wires. This technology is used to an everincreasing extent under the indication “Surface Mounted Device” (SMD).

Capacitors suitable for the SMD technique may be manufactured as a chipcomponent and as a MELF component. Chip components generally havesupporting members in the form of rectangular parallelepipeds which haveend faces suitable for soldering or in the form of flipchips which havea face with both cathode and anode terminals suitable for soldering.MELF (Metal Electrode Face Bonding) components typically start fromcylindrical supporting members having connection caps in which theconnection wires are omitted and the caps themselves are made suitablefor soldering at their surfaces by an electroplating treatment and aresoldered directly with said connection caps to conductor tracks ofprinted circuit boards.

The great advantage of the SMD technology is that extremely high packingdensities of components on the printed circuit boards are possible. Forrealizing ever increasing densities, smaller and smaller componentssuitable for the SMD technique become necessary.

However, SMD technology encounters problems with producing devices withproductivity and uniformity. It can therefore be seen that there is aneed for an improved surface mount MELF capacitor and method for makingthe same.

In addition, current SMD technology may require the manipulation ofindividual capacitors as opposed to using techniques for massmanipulation of capacitors. One particularly useful technique of massmanipulation is through the use of a reel to reel process. Therefore, afurther feature of the present invention is the provision of a capacitorthat is efficiently manufactured using a reel to reel process.

Also, current SMD technology may be improved by the use ofelectrophoretic deposition. Some of the advantages of electrophoreticdeposition include a high coating rate of charged particles upon thesubstrate, a resulting film of charged particles upon the substrate thatis dense and uniform, a thickness of film that is able to be controlledby depositing condition, and a simple process that is easy to scale up.Accordingly, a still further feature of the present invention is theprovision of a method that uses electrophoretic deposition to increasethe capacitor uniformity, tolerance, capacitance and the density pervolume.

It is still a further feature of the present invention to provide asurface mount MELF that is easy to make and economical to manufacture.

The device and method of accomplishing these and other features willbecome apparent from the following description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross sectional view of a surface mount MELF capacitorof the present invention.

FIGS. 2–9 are cross sectional side views of the surface mount MELFcapacitor shown in FIG. 1 at various manufacturing stages.

FIG. 10 is a schematic drawing of a prior art capacitor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described as it applies to the preferredembodiment. It is not intended that the present invention be limited tothe described embodiment. It is intended that the invention cover allalternatives, modifications, and equivalencies which may be includedwithin the spirit and scope of the invention.

FIG. 10 shows a typical prior art capacitor 10. Capacitors are used inmany types of electronic devices. The more popular uses for capacitorsare in personal computers, disk drives, cellular phones, printers, handheld pagers, automobiles and in military equipment.

The capacitor 10, as shown, has two conductors, namely, the tantalumpellet 12 and the manganese dioxide (MnO₂) 16, which is actually asemiconductor. The dielectric film 14 is tantalum pentoxide (Ta₂O₅).When the capacitor 10 is in use, the tantalum pellet 12 is positivelycharged and acts as the anode, and the manganese dioxide 16 isnegatively charged and acts as the cathode. The capacitor also includesa tantalum anode lead wire 18, a metallized outer electrode or silver 20and a layer of carbon 22 inside the outer electrode 20.

The prior art capacitor 10 is usually made by taking tantalum powder andcompressing or compacting into a pellet. The resulting pressed pellet 12then undergoes a sintering process wherein the pellet 12 is heated in avacuum. The heating allows the tantalum particles to stick together sothey can hold the lead wire 18.

After the sintering process, the pellet 12 is typically dipped in anacid solution to form a dielectric film 14 on the outer surface of thepellet 12. The pellet 12 is then subsequently coated with various othermetal-containing materials which form the cathode. Typically, MnO₂ 16 isplaced around the dielectric film 14 which may be followed by the layerof carbon graphite 22 which is painted with silver print 20. Otherconductive polymers such as polypirrolle can also be used in place ofmanganese oxide. The cathode portion ends in a cathode termination.

The lead wire 18 is usually coated with an insulating substance such asTeflon™ (not shown). The lead wire 18 is typically the anodetermination. These terminations can be connected to a circuit board formounting the capacitor 10 in an electrical circuit.

FIG. 1 shows a surface mount MELF capacitor 30 of the present invention.Note that in the figures, for clarity, the various portions of thecapacitors are shown with straight and sharply cornered edges. Theactual capacitors may have slightly rounded corners, etc. In addition,the capacitors have been shown in a standard shape and size; however,the shape and size may vary to include different lengths, widths,heights, size proportions of components, etc.

The capacitor 30 includes a wire 32. The wire 32 is typically made oftantalum. Alternatively, the wire may be made of another valve metal(i.e., Niobium (Nb), Hafnium (Hf), Zirconium (Zr), Titanium (Ti),Vanadium (V), Tungsten (W), Beryllium (Be), or Aluminum (Al)).Alternatively, the wire may be made of a substrate containing a valvemetal (i.e., Ta, Nb, Hf, Zr, Ti, V, W, Be, or Al). The wire ispreferably between 50–100 μm thick. The wire is typically cylindricalwith a circular cross section; however, the wire 32 can be in any shapeand cross section.

A conductive powder element 34 is upon the wire 32. The conductivepowder element may be a valve metal. Alternatively, the conductivepowder element may be a valve metal substrate. The conductive powderelement 34 may have a low capacitor-voltage (CV) (i.e. 10 CV) up to100–150 KCV. The conductive powder element 34 before being placed uponthe wire 32 may be in a form of a powder that is regularly agglomerated,sieved, and/or crushed. The conductive powder element 34 has a densityin the range of 3–8 g/cc when attached to the wire 32 in a layer.

A dielectric film 36 is over the surface of the conductive powderelement 34 and the anode wire 32. The dielectric film 36 is typicallytantalum pentoxide (Ta₂O₅).

A solid electrolyte, i.e. manganese dioxide (MnO₂) or a conductivepolymer is a dielectric film 40. The solid electrolyte impregnatesspaces within the dielectric film 36 coated conductive powder element 34to form the cathode of the capacitor.

A conductive counterelectrode layer overlies the manganese dioxide layer40 and is in electrical continuity with the manganese dioxide layer 40of the capacitor 30. The counterelectrode layer is preferably comprisedof a first sublayer 42 of graphite carbon and an overlayer of metalparticles 44, preferably silver, in a binder or organic resin. Thecounterelectrode layer extends around the cathode end 46 of theconductive powder element 34 as well as helps seal the manganese dioxidelayer 40. The counterelectrode layer overlies substantially all of theside surfaces of the conductive powder element 34 to obtain a capacitorhaving a minimum dissipation factor and ESR, but is maintained separatefrom, and out of electrical continuity with the anode wire 32.

An organic coating or passivation coating 48 is formed over thecounterelectrode layer on the outer perimeter of the conductive powderelement 34 and over the conductive powder element 34 at each end. Acathode ring 54 is bonded in contact with the cathode end 46 of thecounterelectrode layer, thus forming a cathode terminal 56. An anode endcap 58 is bonded to the wire 32 which is in contact with the anode end50 of the conductive powder element 34, thus forming an anode terminal60.

The cathode terminal 56 and the anode terminal 60 are connections thatcan be connected to a circuit board for mounting the capacitor 30 in anelectrical circuit. While the method described below and shown in FIGS.2–9 below is applied to a capacitor, it is also possible to utilize thepresent method for any type of chip component.

FIG. 2 is a side view of a wire 32. The wire is preferably 50–100 μmthick.

As seen in FIG. 3, the conductive powder element 34 is placed upon thewire by electrophoretic deposition that comprises essentially two steps:first, charged particles of powder (0.2–40 μm) in suspension are movedto the wire 32 by applied voltage and second, the particles of powderare deposited (discharged and flocculated) on the wire 32. The resultingfilm of charged particles is the conductive powder element 34 which isdense and uniform.

The next step is to place the wire 32 with conductive powder element 34through a sintering process to heat the conductive powder element 34 ina vacuum. The temperature for this process is between 600–1400° C. fortantalum and niobium. The conductive powder element 34 is held in avacuum at the specified temperature for between about 2–20 minutes andthen cooled in accordance with conventional cooling procedures that arewell known in the art.

As seen in FIG. 4, after the sintering process the conductive powderelement 34 is placed in an oxygen-forming solution such that a thindielectric film 36 is formed. As an example, when using tantalum orniobium powder the thin dielectric film 36 will be tantalum pentoxide orniobium pentoxide.

Next, the cathode portion of the capacitor is formed. Typically,manganese dioxide 40 is placed around the dielectric film 36 which maybe followed by a layer of carbon graphite 42 which is printed withsilver 44. The silver print 44 is comprised of an organic resin heavilyfilled with silver flakes, making it conductive. The first sublayer 42of graphite carbon 42 and the overlayer of metal particles arecollectively called a conductive counterelectrode layer.

As seen in FIG. 6, an insulation or passivation material 46 is placedsurrounding the conductive powder element 34, first and second ends andoutside perimeter, and the exposed portion of the wire 32 side surfaces.

As seen in FIG. 7, the openings for the anode terminal 60 and thecathode terminal 56 are laser opened to expose the wire 32 and theconduction counterelectrode layer, respectively. While laser opening isthe preferred method to expose the conductive surface of the wire 32 andcounterelectrode layer, other techniques could be used. Once the wire isexposed, a silver print 58, 56 can be applied as seen in FIG. 7.

The next step is to cut the surface mount MELF capacitor 30 from theseries into single components. The surface mount MELF capacitor 30 maybe removed from the series a number of ways well known in the art.

While the present invention can be accomplished using the methodsdescribed above, it us understood that various other methods could beused within the spirit and scope of the present invention.

The preferred embodiment of the present invention has been set forth inthe drawings and specification, and although specific terms areemployed, these are used in a generic or descriptive sense only and arenot used for purposes of limitation. Changes in the form and proportionof parts as well as in the substitution of equivalents are contemplatedas circumstances may suggest or render expedient without departing fromthe spirit and scope of the invention as further defined in thefollowing claims.

1. A surface mount MELF capacitor comprising: a wire having oppositefirst and second end surfaces and side surface; a conductive powderelement electrically connected to the wire exposing the second and firstend surfaces, and covering the wire side surface; a first and secondportion of the side surface adjacent the first and second end surfacesis exposed without the conductive powder element; the first portionhaving an inner part and an outer part, insulative material surroundingat least a portion of the conductive powder element, a portion of thewire side surface, and a portion of the inner part; a first terminalformed by a first body of conductive material disposed upon a portion ofthe outer part of the first portion and a portion of the insulatingmaterial; and a second terminal formed by a second body of conductivematerial disposed over the conductive powder element and beingelectrically connected to a second end of the conductive powder element.2. The surface mount MELF capacitor of claim 1 wherein the firstterminal is an anode and the second terminal is a cathode end.
 3. Thesurface mount MELF capacitor of claim 1 wherein the conductive powderelement is made of powder.
 4. The surface mount MELF capacitor of claim3 wherein the powder is from the group consisting of: Ta, Nb, Hf, Zr,Ti, V, W, Be, and Al.
 5. The surface mount MELF capacitor of claim 3wherein the powder is a substrate of a metal from the group consistingof: Ta, Nb, Hf, Zr, Ti, V, W, Be, and Al.
 6. The surface mount MELFcapacitor of claim 3 wherein the powder has been electrophoreticallydeposited upon the wire.
 7. The surface mount MELF capacitor of claim 1wherein the conductive powder element has a density between 3–8 g/cc. 8.The surface mount MELF capacitor of claim 1 wherein the conductivepowder element has a capacitance-voltage between 10 CV and 150 KCV.
 9. Asurface mount MELF capacitor comprising: a wire having opposite firstand second ends and a side surface; a conductive powder element havingbeen electrophoretically deposited around the side surface; a dielectricfilm formed around the conductive powder element; a solid electrolyteformed around the dielectric film; a conductive counterelectrode layerformed around the solid electrolyte; a first and second portion of theside surface adjacent the first and second ends exposed without theconductive powder element, the dielectric film, the solid electrolyte,and the conductive counterelectrode layer; the first portion having aninner and outer part, an insulative layer formed around the conductivecounterelectrode layer, the second portion of the side surface, and theinner part of the first portion; an anode terminal of conductivematerial disposed upon the outer part of the first portion and a portionof the insulating material; an opening formed in the insulative layerexposing the conductive counterelectrode layer, a cathode terminal ofconductive material disposed within the opening.
 10. The surface mountMELF capacitor of claim 9 wherein the conductive counterelectrode layerhas a carbon graphite layer in contact around the solid electrolyte anda silver layer around the carbon graphite layer.
 11. The surface mountMELF capacitor of claim 9 wherein the first and second portion of theside surface are formed by laser cutting through the conductive powderelement, the dielectric film, the solid electrolyte, and the conductivecounterelectrode layer.
 12. The surface mount MELF capacitor of claim 9wherein the insulative layer opening is formed by laser cutting throughthe insulation.
 13. The surface mount MELF capacitor of claim 9 whereinthe anode terminal and cathode terminal are in horizontal alignmentround a circumference outside the insulation layer.
 14. The surfacemount MELF capacitor of claim 9 wherein the conductive powder elementhas a density between 3–8 g/cc.
 15. The surface mount MELF capacitor ofclaim 9 wherein the conductive powder element has a capacitance voltagebetween 10 CV and 150 KCV.
 16. A series of surface mount MELF capacitorsproduced using a reel to reel process, the series comprising: a wirehaving opposite first and second ends and an outer circumference; aplurality of spaced apart surface mount MELF capacitors that have beenformed upon the wire; each surface mount MELF capacitor having: aconductive powder element electrophoretically deposited upon the wireand defined by a first and second portion of exposed wire, the firstportion having an inner part and an outer part; an insulative layerformed around the conductive powder element the second portion ofexposed wire and the inner part of the first portion; an anode terminalof conductive material disposed upon the outer part of the first portionand a portion of the insulating material; a cathode terminal ofconductive material disposed within an opening formed in the insulativelayer and in contact with the conductive powder element.
 17. The seriesof surface mount MELF capacitors of claim 16 further comprising a cutpoint between each surface mount MELF capacitor.